This invention relates to foamed, lightweight, electrically conductive, polymeric materials. Electroconductive foams have widespread application in the packaging of electronic devices due in part, to the ability of such foams to dissipate static electricity. As electronic circuitry is miniaturized, it becomes increasingly susceptible to damage from electrostatic discharge (ESD) since the level of voltage which may permanently impair or destroy circuitry decreases as the physical size of circuitry is reduced. Thus, the range of voltages which may damage circuitry is now typically in the realm of voltages associated with ESD. Damage from ESD has been estimated to cost the electronics industry billions of dollars annually, and is expected to increase as further circuitry miniaturization occurs.
There are primarily two mechanisms by which materials conduct electricity; ionic conduction and metallic conduction. Typical metallic conductors include metals (e.g. in the form of wire, films or fibers) and conductive carbon black. Metallic conduction requires the presence of an electrically conductive pathway through the material. Continuity is a critical factor in establishing metallic conduction. That is, physical contact or very near proximity between the conductive particles must occur for electrons to pass through the material. Thus, in a polymer matrix loaded with carbon black particles, the particles must touch or nearly touch one another in order to provide an electrically conductive pathway through the material.
Prior artisans have utilized foams containing electrically conductive particles such as carbon black dispersed throughout the foam. However in order to obtain an adequately conducting foam, carbon black concentrations in the range of 10 to 25 percent by weight (based upon the total weight of the foam) are often required. Carbon black loadings up to 40 percent and higher have even been described as in U.S. Pat. Nos. 4,231,901 to Berbeco and 4,481,131 to Kawai et al. It is only at such high concentrations that the particles contact one another or are sufficiently close to one another to provide an electrically conductive pathway through the foam matrix.
It is not desirable to have such high concentrations of conductive particles in foams for several reasons. First, the higher the concentration of particles in the foam, the greater the cost of materials and processing. Second, when attempting to foam polymeric resins containing such high particle concentrations, it is difficult to extrude the resin due to the resin's poor melt viscoelasticity and the tendency for particle agglomeration. Third, the resulting foams have relatively high densities rendering them undesirable for packaging and shipping applications. Fourth, the particles near the surface of these foams tend to slough from the foam surface during fabrication and handling, thereby increasing the risk of contamination of electronic devices if the foam is used for packaging or in the vicinity of sensitive components.
The second mechanism by which a material may conduct electricity is ionic conduction. These systems rely on ionic charge carriers for electron transfer, and as such the charge carrier population, capacity, and velocity are critical factors which affect the conductivity of the material under consideration. Moreover, many of these factors are further dependent upon other criteria. For instance, the population or concentration of charge carriers depends upon the extent of dispersion, distribution and solubilization of the particular ionizable compound(s) in the host material. In addition to the complexity and unpredictability of ionic conduction, such systems are much slower than metallic systems since electron transfer occurs via ionic carriers as opposed to the near speed of light displacement of electrons along the conductive pathway in metallic conduction systems.
An example of ionic conduction in polymeric materials is the application of topical treatments to the outer surface of the polymeric material, or the use of additives which migrate to the material surface to provide electrical conductivity on the surface or skin of the material. Examples of such surface active additives include quaternary ammonium salts, or other fatty amines, glycols, and sulfonates. For systems of this type, the conductivity properties as measured along the outer surface of the polymeric material are often very good. However, such surface active additives do not affect the volume resistivity of the material, i.e. the conductivity as measured across a cross section of the material. Moreover, foams having such surface active additives suffer from a variety of drawbacks such as; the conductivity of the foamed material tends to decrease over time, the conductivity is often significantly dependent upon humidity, the degree of conductivity is typically nonuniform, and the foam tends to be corrosive to sensitive electronics due to the presence of the additive(s).
Some foams contain a hygroscopic antistatic additive which functions to reduce surface resistivity by migrating to the foam surface and attracting moisture from the surroundings. Antistatic properties of the foam skin are excellent, however the conductivity as measured across a cut surface of the foam is only marginal. Since moisture is one of the essential components in forming a thin electrolyte layer on the material outer surface, antistatic foams made with the additive may perform poorly at low relative humidity. Additionally, the additive may cause contamination of adjacent devices or materials and be incompatible with some polymeric resins.
Prior artisans have attempted to avoid many of the problems encountered in the prior art associated with ionic conduction systems by utilizing complexes of ionizable salts and oxygen-containing polymeric materials to achieve electrical conductivity, such as described in U.S. Pat. Nos. 4,617,325 and 4,618,630 to Knobel et al., assigned to the Dow Chemical Co. and 4,359,411 to Kim et al. Although such compositions generally provide improved electrical conductivity and moisture dependency, such compositions do not exhibit surface resistivities of less than 10.sup.10 ohms per square.
Thus, the need exists for a lightweight foam which has a surface resistivity less than 10.sup.10 ohms per square, and which has a relatively low concentration of conductive particles thereby avoiding the problems experienced with prior art compositions containing relatively high concentrations of conductive particles such as relatively high material and processing costs, difficult manufacturing aspects, relatively high densities even after foaming, and detrimental sloughing of conductive particles from the foam surface.
Moreover, it has been found that it is difficult if not impossible to produce foams having large cross-sectional areas by extrusion processes if the resin contains a relatively high concentration of carbon black particles. Thus, the need exists for a method of producing an electroconductive foam having a surface resistivity of less than 10.sup.10 ohms per square by an extrusion process.
In addition, the need exists for an electroconductive foam which avoids many of the problems encountered by prior artisans when utilizing ionic conduction systems in foams such as decreasing conductivity over time, significant dependence of conductivity upon humidity, nonuniform conductivity, and corrosiveness of such foams due to the relatively high levels of additives in the foams.